NBC News correspondent Richard Engel is celebrating a milestone for his 3-year-old son with an emotional essay.

Engel, 45, opened up on the "Today" show Friday and penned a loving essay about his son Henry finally saying "Dada" after being diagnosed with Rett Syndrome, a rare genetic neurological disorder that leads to severe physical and cognitive impairments.

I finally got a “Dada.” It was a long time coming, which made it all the sweeter — an unexpected reward.

Dada. Dada. Dada!

"Our son Henry is 3 and a half years old and has special needs," Engel said in the essay. "His physical and mental development aren’t on pace with other children. So when I got the Dada from him, I went a little wild."

The NBC News chief foreign correspondent said that the revelation happened when he returned from a work trip to Syria.

"My wife Mary had been telling Henry I was coming home, but because of his condition it’s hard to know what’s sinking in," Engel wrote in the essay, "He still struggles with basic things like picking up a ball with both hands."

Engel continued, recounting the sweet story of the moment his son said "Dada."

"As I was singing good morning to Henry and reconnecting after a long trip away, he looked at me, locked eye-contact and said, clear as any word, 'Dada,'" Engel wrote.

He didn’t just say it once, but two or three times. There was an urgency and excitement to it. Dada. Dada. Dada!

'My world lit up'

Engel also left parents with a message about the importance of the moment.

"To parents with typically developing children, a little Dada may not seem like a big deal," he wrote. "But for me it was a validation, an acknowledgement that he’s in there, knows me, knows that his mother and I are forces for good in his life, and above all, that he loves us."

Engel told Hoda Kotb on "Today" that his son saying "Dada" for the first time "was something that I'd been waiting for for years."

“A lot of parents, it happens naturally, it happens early on. Henry’s situation is different," he said. “My world lit up.”

Our own @richardengel’s 3-year-old son Henry has Rett Syndrome. There are hopes for a possible cure for this debilitating genetic condition. Henry’s doctor believes that Henry could hold the key she’s been looking for. pic.twitter.com/KWFBKmgY6c